Electrolytic production of chromium hydride



April 21, 1953 c s v L 2,635,993

ELECTROLYTIC PRODUCTION OF CHROMIUM HYDRIDE Filed 001,. 16, 1948 CURRENT DENSITY VS. Cr 0 CONCENTRATION IN BATH FOR PRODUCING FACE'CENTERED CUBIC CHROMIUM HYDRIDE 500 700 I 900 I100 I300 I500 Cr0 IN g./|.

JNVENTOR.

Cloyd A. Sncwely Patented Apr. 21, 1953 ELECTROLYTIC PRODUCTION OF CHROMIUM HYDRIDE Cloyd A. Snavely, Columbus, Ohio, assignor, by mesne assignments, to The Batte'lle Development CorporationyColumbus, Ohio, a corporation of Delaware Application October 16, 194s,"seria1Nae-1,987

2 Claims.

This invention relates to a new method for making chromium hydride having a face-centered, cubic structure.

Metal hydrides have unusual and distinct properties which make them useful for numerousindustrial and special applications. On being decomposed, they provide a source of chemically pure hydrogen which, released in the nascent or active atomic form, is highly reducing. This active hydrogen may be utilized to reduce the oxidized forms of metals to their metallic state, or to reduce various organic or inorganic com pounds. After release of the hydrogen, the metal powder which remains is often in a very pure. state. Furthermore, metal hydrides may be used'in formulating compacts in powder metallurgy where the-hydrogen released during subsequent sintering operations acts as a protective atmosphere to prevent oxidation.

The usual methods of producing metal hydrides have involved passing ure hydrogen gas through metal powder under carefully controlled conditions. These procedures have required a source of pure metal powder. Moreover, such methods have necessitated large supplies of hydrogen and extensive plant equipment for heating the metal powder and forcing the hydrogen gas through it. Certain of the metal hydrides which are not readily prepared by this method can be produced by reacting metal oxides with the hydrides of compound which is a source of active hydrogen 4 and pure metal at low temperatures.

It is another object of this invention to provide face-centered, cubic, chromium hydride.

It is a further object of this invention to prochromium hydride.

It has been found that, if the current density and the concentration of chromium trioxide are maintainedin acertain relationship, it is possible rent density and the concentration of chromium trioxide is illustrated by the figure where the concentration of chromium trioxide in grams per liter is plotted against current density in amperes per square foot. Outside of the band of the figure whose limits are defined by A and B, it has been found that only hydrolysis products, metallic chromium .or hydrogen will be produced. In addition to maintaining this important relationship between "the current density and the concentrati'on of chromium trioxide, the bath must be operated at temperatures of from 0 C. to 25 C.,have an acid ratio of from 250:1 to 350:1, and contain sufli/cient reducing agent to provide from 3% to 20% trivalent chromium ion in the bath fromthe'reduction of hexavalent chromium 1011.

1 A, bath having aconcentration of from 500 grams per liter of chromium trioxide to a saturated solution (about 1300 gms/l.) and operated at from 28 to 430 amps/sq. it. will produce the hydride. Where the bath is operated at about 28 amps/sq. ft; the solution should be saturated with chromium trioxide, and less than 28 amps/sq. ft. at this concentration will not produce the hydride. 0n the other hand where the concentration of chromium trioxide in the bath is around"500gms./l., the current density should be about 430 amps/sq. it. At concentrations less than about 500 .gms/l. of chromium trioxide, still higher current densities will not producethe hydride. The curve of the figure is shown as a broad band rather than as a narrow line to providefor slightvariations in the bath composition and operating conditions which occur due to the temperature at which the bath is operated and the concentration of the reducing agent employed to reduce part of the hexa valent ion to the trivalent state. For example, ifthe process of plating is conducted where the concentration of chromium trioxide is 1000 grams per liter, the current density in the cell may vary from about 85 to about 145 .amps..persquare foot as ated by therange defined by the points vide a process for making face-centered, cubic,

to plate face-centered, cubic, chromium hydride 5 on a cathode from a low-temperature, aqueous chromic acid plating bath having an appreciable quantity of trivalent chromium ion therein and a high acid ratio.

The important relationship between the curwand bonthe band shown in the figure. In general, .it 'has beeniound that, the other conditions "of the'plating process remaining constant (concentration of sulfuric acid, reducin agent, hexavalent chromium ion, trivalent chromium ion, and temperature), anincrease in concentrae tion of chromium triox'i'de requires a decrease in current density if face-centered, cubic, chromium hydrideis to be plated on the cathode.

The sulfate ion cohcentrationshould be low, or, conversely, the acid ratio (GIGS/s04) shouldbe high. Sufficient sulfuric acid should be added to the bath to give an acid ratio of from about 250/1 to 350/1. It is preferred to utilize a bath having a chromium trioxide to sulfate ion ratio of about 300 to 1. In place of the sulfuric acid, other compounds which furnish-the S04 radical and which do not adversely effect @the electrolytic bath, may be employed. Suitable compounds of this nature are sodium sulfate, calcium sulfate, and chromium sulfate.

Trivalent chromium ions which are needed for the successful deposition of face-centered, cubic, chromium hydride from this electrolytic bath are obtained from the reduction vof .hexavalent chromium ions. It has been found that the bath should have a concentration of from 3% to 20% trivalent chromium ion for successful production of the hydride. To reduce the hexavalent ion to the trivalent state it is necessary to use a suitable reducing agent. Such reducing agents are well known in the art and it is only necessary that harmful by-products are not thereby introduced into the bath by cathodic reduction or by direct addition. Cane sugar has been found to be a very satisfactory reducing agent, as is hydrogen peroxide.

From about 5 to 40 grams of sugar per liter is satisfactory to reduce enough chromium to the trivalent state and to provide a sufficient concentration of the ion in the electrolyte. It is preferred to employ a'concentration of sugar in the amount of from 10 to grams perliter.

While the electrolytic process is being carried on, the bath should be maintained at a low temperature. The temperature of the bath should be held below C. and above a temperature which would freeze the bath. Satisfactory deposits have been obtained when the bathtemperature was between 0? C. and about 12 C. Excellent results have been achieved with a bath held at a temperature of between2 C. and 8 C.

Insoluble anodes are best for use in plating the hydride. Antimonial lead anodes are preferred as they are easily obtained and are satisfactorily inert. Pure lead or iron anodes may be'used but are less satisfactory, because both are less inert than lead alloy anodes to the chemical environment prevailing in the bath. Iron anodes also seem to lack the desirable properties associated with the lead oxide coatings formed on lead or lead alloy anodes during electrolysis of the bath.

The cathode on which the hydride is plated may consist of copper, brass, steel, or any of the metals which are commonly plated with metallic chromium.

The plating time is not believed critical, for it only determines the thickness of the deposit on the cathode. Some plates have been produced in a few minutes; Thick deposits were obtained after the process had been carried on for several hours.

The hydride deposited on the'cathode is taken directly from the plating bath with no anodic replenishment; Periodic additions of chromium trioxide to the bath are therefore necessary. The sulfate concentration of the bath is not similarly reduced, the only appreciable losses of this constituent being the amount removed from the bath in the solution film clinging to the plated work. Such losses are referred to, as dragout losses. Periodic checks are necessary to keep the acid ratio within the desired limits.

' The following examples are set forth to more readily enable those skilled in the art to practice the invention: p

Example 1 One thousand grams of chromium trioxide were dissolved in a liter of water. 3.3 grams of sulfuric acid were then added and mixed into the solution. 20 grams of cane sugar in the form of an aqueous syrup were then slowly added to this solution. A strong reaction proceeded, the sugar reducing much of the chromium, which was in the hexavalent state, to the trivalent state. After the reaction had subsided the bath was tested or titrated, and it was found that the trivalent chromium content of the bath was now approximately 12% of the total chromium present. The electrolytic bath was then refrigerated to 5 C., plus or minus 4 degrees. Electrodes of lead and copper were then placed in the bath, the anode being lead and the cathode being copper. An electriccurrent was then passed through the electrolytic bath which was sufficient to create a current density in the cell of 115 amps per square foot. After seven hours a deposit on the cathode was obtained which was dark gray and lustrous in color and extremely scratch resistant. On X-ray analysis it was found to have a facecentered, cubic structure. Chemical and physical analyses were performed on this deposit which showed that it contained only chromium and hydrogen.

Example 2 A solution was prepared similar to that shown in Example 1 containing 911 grams per liter of chromium trioxide and sufficient sulfuric acid to give a 300:1 acid ratio. 10 grams per liter of cane sugar were added to this bath. On titrating it was found that approximately 5% of the chro mium was reduced to the trivalent state. Samplesplated from this bath at temperatures below 5 C. and at a current density of approximately amps per square foot over a 17-hour interval were X-rayed. The X-ray data showed this ma-- man in Electronics, April 1945, page 132. Ape proximately 250 samples of plate were produced during this investigation and most of these were X-rayed from 1 to 10 times to identify their initial structures and any structural changes occurring after the initial determination.

TABLE 1 Cu Radiation-20 (in Degrees) Sample Body-centered cubic chromium .1 X X Failzle-centeret l1 cgbi c, X N

0 romium y n e X Sample as plated... X X Sample after heating- X X h 1. Values of 20 for B. G. 0. structure were calcul t d f llshed lattice constants. .26 values for F. C. C. stru ctgre 222 1:22- mgntazlavaltues obtameg thelpresent work.

. ea mg consls e in p acing sam les in an o n h'ld O. ior an hour to decompose the hydrid structure 6 at arose-gees From thesedimensionsit was possible 1130100111 pute' the lattice parameter ofithe :faceecentered, cubic structure which was found tokbe .llo=3. 8.5 angstromunits. The distances between centers of adjacent chromium atoms wasialso :computed for this structure and found to be approximately 2.712 uangstro m. units. The lattice parameter, however, of body-centered, cubic chromium. is givenin the literature-as wo=2.87 (6) and the distance between centers of. adjacent atoms is 2.492 (9),.A.

Hydrogen .atoms are believed .to occupy the second ..;largest openings in 'thelattice. .iMetalhydrogen combinations of the general formula MHftO. MHz.crystallize in the, face-centered, .cubic atomic arrangement with metal .atoms at the normal corner and face-centeredpositions or the unit cell. Thehydrogenzatoms then occupy half of the-second largest interstitial openings to .form a zinc blende type oi structure for the vformula MH .a-s proposed for alloys by Westgren, Jnl. Franklin Inst., 21211931) .577, or-all of the sec-- ond largestinterstitial openings to form afiuorite type of structure for the formula MHa also as proposed .by Westgren.

The atomic radius of chromium in .the. bodycentered cubic arrangement is l.24+A, and the Goldschmidt correction, Z. Physika1Cl1em., 133 (1928) 397, applied to this requires a radius of 1.28+A for atomic arrangements of coordination number 12. If atoms of radius 1.28+A are considered to be the metal atoms in a face-centered, cubic lattice of parameter 3.85 A., the largest opening in the structure is at the center of the unit cube. This opening is 1.29 A. in smallest diameter. The second largest openings are 0.78 A. in smallest diameter and are centered on positions in the unit cube of A A, and the seven corresponding positions in the lattice. The diameter of the interstitial hydrogen atoms in facecentered, cubic structures is given in the literature as being between 0.80 A. and 1.00 A. A hydrogen atom of this size accounts for the lattice parameter of the face-centered, cubic, chromium plate. Thus, the face-centered, cubic plate is in reality a hydride of the zinc blende type of structure. Since the fluorite type structure is only a more filled-up version of the zinc blende structure, it is likely that any combination of chromium-hydrogen atoms from 50 atomic per cent Cr-50 atomic per cent H (zinc blende type) to 33 atomic per cent Cr67 atomic per cent H (fluorite type) should be possible.

To provide further proof that the face-centered cubic, chromium plates were hydrides, some hydrogen extraction tests were undertaken. A sample of face-centered, cubic, chromium hydride plate was produced from a bath containing 1021 grams per liter of chromium trioxide having a 300:1 acid ratio, and with 7.4 per cent of the total bath chromium in the trivalent state from the reduction of hexavalent chromium ion by 20 grams per liter of cane sugar. The bath temperature was approximately 0 to 4 C., the current density was 115 amps. per sq. ft., and the plating time was 7%.; hours. The backing for this plate was copper foil, 0.002-inch thick. Upon removal from the plating bath, the sample was quickly rinsed, sponged dry with a towel and placed under mercury in an inverted test tube. This operation required approximately two minutes. Gas was evolved from the specimen for less than twenty-four hours at an appreciable rate; no further cold evolution being noted in three days. The sample was then transferred to a gas 61 extraction apparatus wherezita was-heated to 650 C. and held at this temperatureasfor 30 minutes while under a vacuum. Theugasextracted by this :treatment and ffrom "the cold evolution was analyzed 1 andgiound :to be 1 pure hydrogen. Calculated. at standard. temperature and pressure, the :cold evolutiongashad .a volume of 1:41 m1, and. the warm extractiongas-hadta volume of 10:80. ml.,.giv-ing .a total gas content .of 12.21 ml. forxithe sample; This amount iof.hydrogen was considered .to the evolved entirely "fromithe chromium plate, :as previous experience has shown that .axnegligible amount of hydrogen .may .be held in a copperdiscmeasuring only /2 .inchzin diameteriand 0.002 zinchrin thickness. Theapparatus aused toextract the hydrogen :gas was similar;to .that; described by Newell, Journal of the Iron and. Steel Institute, 141 l (1940) 243.

. ;The coppertbacking was stripped off thezsample in nitric acid and the residue was weighed. It was then treated with hydrochloric acid which dissolved some of the chromium. The undissolved residue was a green powder, and .it was concludedthatit wwas ,chromic oxide .(CrzOs) which -was then Iusedwith potassium ubisulfiate,

some residue remaining. A sodium peroxide fusion treatment served to dissolve this residue. The entire lot of dissolved chromium was then oxidized with sodium peroxide and titrated by the iodine-thiosulphate method, from which the amount of chromium in the sample was found to be 0.0329 gram. Since the weight of the sample was 0.0511 gram, it is evident that roughly half of the sample was in the form of chromic oxide (CrzOs) after the high-temperature extraction treatment.

The chromium atom to hydrogen atom ratio calculated from the above data is 1:1.7. This result entirely agrees with the expectations, as a chromiumzhydrogen ratio anywhere within the limits of 1:1 and 1:2 would satisfy the requirements previously computed with X-ray data for the face-centered, cubic structure. It is, therefore, believed that face-centered, cubic, chmmium hydride has the general formula 'CrHum), wherein N ranges from 0 to 1.

In summary this invention discloses a new and novel process for plating chromium hydride on a cathode from a low temperature chromic acid bath, containing a minor amount of sulfuric acid and an appreciable quantity of trivalent chromium ion, by maintaining a definite relationship between the chromium trioxide and the current density. This relationship is indicated by the shaded area of the curve defined by the limits A and B, of the figure. In general, within the ranges indicated, the current density will vary inversely with the concentration of chromium trioxide in the bath. The method disclosed herein is extremely easy to perform and does not demand the use of involved techniques nor complicated machinery as compared to known processes for manufacturing other metallic hydrides. Furthermore, face-centered, cubic, chromium hydride which is produced by the method of this invention provides a ready source of nascent hydrogen for reducing or catalytic reactions at low temperatures unlike other metallic hydrides, for it has the property of decomposing slowly at atmospheric temperatures and rapidly at only slightly elevated temperatures. It is, however, brittle and may be subjected to comminution to produce a very fine powder which is highly suitable for use in making up powder compacts; and it is also a source of pure, metallic chromium.

What is claimed is:

1. The method of producing essentially pure face-centered cubic, chromium hydride, which comprises passing an electric current from an anode to a cathode through an electrolytic bath at a temperature of from C. to 12 C., said bath consisting of an aqueous solution of chromium trioxide, from to 40 grams per liter of sugar, a sufficient amount of a compound furnishing the S04 radical selected from the group consisting of sulfuric acid, sodium sulfate, calcium sulfate, and chromium sulfate to give a chromium trioxide to acid radical ratio of 300 to 1, While maintaining the current density of the cell from 105 to 125 amperes per square foot with the concentration of chromium trioxide in the substantially inverse ratio of 1100 to 900 grams per liter.

2. A composition of matter useful in the method of producing essentially pure face-centered cubic, chromium hydride, consisting of an aqueous solution of chromium trioxide, from 5 to 40 grams per liter of sugar, and a suflicient amount of a compound furnishing the S04 radi- References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,802,463 Fink Apr. 28, 1931 1,842,535 Brode et a1 Jan. 26, 1932 1,967,789 Sohn July 24, 1934 2,410,242 Schulein Oct. 29, 1946 2,461,661 Schlesinger et a1. Feb. 15, 1949 OTHER REFERENCES Makariewa et al., Zeitschrift fiir Elektrochemie, vol. 41 (1935), pp. 623-631.

Wood, London, Edinburgh and Dublin Philosophical Magazine and Journal of Science, vol. 12 (1931), pp. 853-864; vol. 24 (1947), pp. 772-776.

Snavely, Transactions of the Electrochemical Society, vol. 92 (1948), pp. 541-545. 

1. THE METHOD OF PRODUCING ESSENTIALLY PURE FACE-CENTERED CUBIC, CHROMIUM HYDRIDE, WHICH COMPRISES PASSING AN ELECTRIC CURRENT FROM AN ANODE TO A CATHODE THROUGH AN ELECTROLYTIC BATH AT A TEMPERATURE OF FROM 0* C. TO 12* C., SAID BATH CONSISTING OF AN AQUEOUS SOLUTION OF CHROMIUM TRIOXIDE, FROM 5 TO 40 GRAMS PER LITER OF SUGAR, A SUFFICIENT AMOUNT OF A COMPOUND FURNISHING THE SO4 RADICAL SELECTED FROM THE GROUP CONSISTING OF SULFURIC ACID, SODIUM SULFATE, CALCIUM SULFATE, AND CHROMIUM SULFATE TO GIVE A CHROMIUM TRIOXIDE TO ACID RADICAL RATIO OF 300 TO 1, WHILE MAINTAINING THE CURRENT DENSITY OF THE CELL FROM 105 TO 125 AMPERES PER SQUARE FOOT WITH THE CONCENTRATION OF CHROMIUM TRIOXIDE IN THE SUBSTANTIALLY INVERSE RATIO OF 1100 TO 900 GRAMS PER LITER. 